[libvirt] [PATCH 3/3] maint: Avoid -Ignulib on standalone examples

[Ján Tomko]

On Fri, Jan 04, 2019 at 02:12:20PM -0600, Eric Blake wrote:
>Travis shows that clang (but not gcc) fails to build after our
>recent gnulib updates, due to:
>

This is a BSD vs Linux thing, we use Travis to get Mac OS X coverage.
I regulary build with clang and did not hit such error.

>  CC       domtop/domtop.o
>In file included from dommigrate/dommigrate.c:26:
>In file included from ../gnulib/lib/stdlib.h:100:
>In file included from ../gnulib/lib/unistd.h:40:
>In file included from /usr/include/unistd.h:638:
>In file included from ../gnulib/lib/sys/select.h:110:
>In file included from ../gnulib/lib/signal.h:67:
>../gnulib/lib/pthread.h:74:3: error: "Please include config.h first."
> #error "Please include config.h first."
>

The reason for this change is gnulib commit 6954995d
https://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=6954995d
which started including unistd.h through stdlib.h for some platforms,
thus requiring us to include config.h.

I did not notice this patch before pushing my "include config.h in
examples" solution:
https://libvirt.org/git/?p=libvirt.git;a=object;h=6954995d

>This probably stems from gcc and clang having a subtle difference in
>-isystem vs. -I include behaviors on #include_next, where that difference
>then results in clang trying to pick up same-named gnulib headers merely
>because their directory was in the search path, while gcc did not hit
>that problem.
>

IIUC for both compilers the expected behavior is to include gnulib
headers, otherwise there's no point in having gnulib at all.

>But for our examples that are intended to be standalone, we want to
>work with the bare-bones libc, without any interference from our
><config.h> or from gnulib.  The easiest way to do that is by using
>per-binary CPPFLAGS, and including gnulib headers only for example
>binaries that still use <config.h>.
>
>Signed-off-by: Eric Blake <eblake at redhat.com>
>
>---
>I don't have a local clang setup, so I'm relying on CI testing to
>prove whether this fixes the build failures that Travis reported...
>---
> examples/Makefile.am | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>

Having standalone examples is a nicer soultion than mine.
However, with my commit reverted, tuning examples/admin/logging.c is
needed to make the build pass on FreeBSD for me:
diff --git a/examples/admin/logging.c b/examples/admin/logging.c
index dc1b23aab5..1e72cdd601 100644
--- a/examples/admin/logging.c
+++ b/examples/admin/logging.c
@@ -1,8 +1,8 @@
+#include "config.h"
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdbool.h>

-#include "config.h"
 #include <unistd.h>
 #include <libvirt/libvirt-admin.h>
 #include <libvirt/virterror.h>

This makes me think that the remaining examples don't really need
gnulib, they just include <config.h> to make compilation pass while
including <unistd.h>.

Jano
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: not available
URL: <http://listman.redhat.com/archives/libvir-list/attachments/20190107/2a87c211/attachment-0001.sig>